The invention relates to laser projection video screen systems and methods employing a pulsed laser source and holographic projection screens.
It has been difficult to project viewable images onto any conventional front projection screen by video projection devices powered by incandescent light sources such as CRT projectors or LCD/light valve projectors with xenon/metal halide lamps under extremely high ambient lighting conditions (outdoors in daytime, for example).
There are two problems. First, a traditional, white, front projection screen returns projected light in a random manner and is referred to as a Lambertian Scatterer with the brightness of the image appearing the same, regardless of the viewer location. This white front projection screen is typically used as a reference point, so that if a screen is able to return a projected image in a more spatially selective manner, then the screen appears brighter and is said to have xe2x80x9cagainxe2x80x9d. The typical white front projection screen is considered to have a gain of 1; whereas, a front projection screen having more sophisticated structures that are designed to limit the returning projected image light to a specified range of horizontal and vertical direction by use of glass beads or other materials with known scattering angles is considered to have a higher gain. The gain of the screen may be a critical component to reproduce the projected image with enough contrast. When the traditional front projection screen is used in bright ambient light conditions, it will reflect back not only the projected images, but also a large portion of undesirable bright ambient light toward the direction of the viewer; therefore, high picture contrast cannot be attained.
A second problem is that the conventional video projection device is powered by an incandescent light source. Since the incandescent light source produces incoherent light rays, they have more chance of being dispersed by random scattering as they travel through the air from the projection device to the screen. The longer the projection throw distance is, the more image dispersion they suffer.
Laser video projection systems with projection screen surfaces are described in U.S. Pat. No. 4,720,747, issued Jan. 19, 1988; U.S. Pat. No. 4,851,918, issued Jul. 25, 1989; U.S. Pat. No. 5,253,073, issued Oct. 12, 1993; and U.S. Pat. No. 5,311,321, issued May 10, 1994, all hereby incorporated by reference.
It is desirable to provide an improved video projection screen, system and method to minimize image dispersion between the projector and screen and to provide screen designs which reflect back the projected image in the viewer direction.
The invention relates to a holographic projection screen and to laser video projection systems and methods employing the holographic screens.
The laser video projection system of the invention comprises a video projection device employing, for example, red (R), green (G) and blue (B) monochromatic laser light sources to form a projected full color video image, and which system includes a projection screen with a holographic pattern on the screen surface which reflects back the projected image in a selected direction, and transmits the majority of ambient light through the screen to provide a high video picture contrast on the screen.
The method comprises projecting a laser video image, typically employing R,G and B pulsed lasers to provide a full color video image onto a projection screen with a selected holographic design, in one or multiple layers on the screen surface to reflect back substantially the full color image to a viewer and to transmit the majority of ambient light through the projection screen.
A viewable projected image onto the screen is achieved by front projection format under high ambient light conditions by:
a) a video projection device powered by coherent/laser light sources (R,G,B) to minimize the image dispersion between the projector and the screen; and
b) a special front projection screen design which only reflects back the projected image to the direction of the viewers, and not to the surrounding areas where no viewer will be (highly directional design), and transmits the majority of the ambient light through the screen so that high picture contrast can be achieved.
This invention embodies two different front projection screen designs incorporating holographic patterns which can be used beneficially with video projection devices powered by laser light sources (R,G,B).
Because a full color video projection device powered by laser light sources (R,G,B) produces specific monochromatic wavelengths of red, green and blue light, it is ideal to construct reflective viewing screens with holographic patterns that will reflect back only those wavelengths of red, green and blue used in the laser video projection device.
The first design is a diffusely-reflecting holographic screen with exceptionally high gain (i.e., well defined viewing cone) which will be best suited for uses under high ambient light conditions. This screen design reflects only the specific monochromatic wavelengths of red, green and blue used in the laser video projection device, therefore, it will not be optically usable with other conventional video projection devices powered by incandescent light sources.
Because the holographic patterns constructed on the screen surface only reflect the specific monochromatic wavelengths of red, green and blue back to the viewers, all other wavelengths from the ambient light will pass through the screen. This will help to increase image contrast and thus make the image much easier to view under high ambient light conditions.
The high transmission of visible wavelengths, except for the specific monochromatic ones generated by the laser video projection device, means that the screen could offer considerable xe2x80x9csee-throughxe2x80x9d features for blending the projected images with real background scenes behind the screen.
Holographic pattern is constructed to direct incoming specific monochromatic light from the laser video projection device into predetermined horizontal and vertical energy distribution zones, thus, this screen produces very bright images by virtue of shaping most of the projected R,G,B laser image light into very well defined, narrow viewing cones.
The method used in the recording of the holographic patterns ensures that there is maximum of diffraction at the specific monochromatic wavelengths from the laser video projection device, so that high reflection of those wavelengths towards the viewer or other direction is achieved.
Mass production of this screen is available once a printing process has been established for this type of hologram. This technique literally uses a modified optical contact printing process to bring the hologram onto thin layers of photopolymerizable plastic materials (typically less than 0.0002xe2x80x3 thick) supported on polycarbonate or polyester film. The holographic pattern is transferred at high speed onto the film, and the completed screen itself ends up on a roll from which the user can cut a piece to the unit size (typically 40xe2x80x3 wide by 80xe2x80x3 high). When a larger screen is required, it can be constructed by tiling smaller hologram units with nearly invisible seams. The hologram may consist of a single diffracting layer or a sandwich of two or three such layers, each optimized for separated wavelength regions.
In another embodiment a second design is a multi-layered holographic screen comprised of a diffusely-reflecting layer, as discussed above, plus an additional layer with diffracting holographic patterns which direct the reflected R,G,B laser images by the diffusely-reflecting layer into pre-determined selected xe2x80x9cleftxe2x80x9d and xe2x80x9crightxe2x80x9d viewing zones.
Two video projection devices powered by R,G,B laser light sources receive stereoscopic video input signals derived from two displaced cameras. The resulting xe2x80x9cleftxe2x80x9d and xe2x80x9crightxe2x80x9d images are front projected onto the multi-layer holographic screen described herein.
Alternatively, when the laser video projection device is similar to the one disclosed in U.S. Pat. No. 4,720,747, two independent transducers attached on a single acousto-optic cell receive the stereoscopic video input signals described above, and the acousto-optic cell is then illuminated by two thin, well-collimated lines generated by a single pulse from a source laser, it will result in two images, a xe2x80x9cleftxe2x80x9d and a xe2x80x9crightxe2x80x9d stereoscopic image. These xe2x80x9cleftxe2x80x9d and xe2x80x9crightxe2x80x9d images are relayed to two separate output optic channels, each of which consists of a vertical scanner and a set of projection optics that focus xe2x80x9cleftxe2x80x9d and xe2x80x9crightxe2x80x9d images onto the multi-layer holographic screen. This method is more attractive than the one described above, because a single laser video projector can generate both of the required stereoscopic images.
The first layer of the multi-layer holographic screen is made of transparent plastic material (either polycarbonate or polyester film) having diffracting holographic patterns printed on the back surface; therefore, the projected R,G,B laser images will transmit through the first layer.
The second layer is a diffusely-reflecting holographic screen; thus, the image will be formed and a majority of the R,G,B laser image light will be reflected back toward the viewer. Then, the diffracting holographic patterns printed on the back of the first layer will direct the reflected stereoscopic R,G.,B laser images into pre-determined xe2x80x9cleftxe2x80x9d and xe2x80x9crightxe2x80x9d viewing zones as the reflected images pass through the first layer, creating the effect of a 3-dimensional (xe2x80x9c3Dxe2x80x9d) image to the viewer.
This multi-layer, three dimensional, holographic screen design will pass all the other wavelengths from the ambient light source through the screen, similar to the first holographic screen, reflecting back only the red, green and blue laser wavelengths from the laser video projector used, which will help increase the image contrast ratio.
This multi-layer, three dimensional holographic screen can be mass produced in a similar manner (i.e., contact-copying process) as the first holographic screen with high gain.
The holographic projection screen is particularly adapted for use in pulsed laser video systems and methods; however, the holographic projection screen may also be employed with, and has advantages with, other light image projection systems and methods incorporating monochromatic or semi-monochromatic R,G,B light sources.
The invention will be described for the purpose of illustration only in connection with certain illustrated embodiments; however, it is recognized that various changes, modifications, additions and improvements may be made in the illustrative embodiments without departing from the scope of the invention.